1. Field of the Invention
This invention relates to a limiter circuit which clamps an input signal to within certain limits, specifically to a limiter circuit suitable for a TV receiver to generate a sawtooth wave used for vertical deflection.
2. Description of the Related Art
A TV receiver of conventional art generates the sawtooth wave based on a vertical synchronizing signal, amplifies it, shapes the waveform and provides a vertical deflecting coil with the sawtooth wave current. The amount of vertical deflection can be modified by the amplitude of the sawtooth wave. Recent TV receivers have a zoom mode in which a display screen is zoomed in.
FIG. 2A and FIG. 2B are to explain the zoom mode of the TV receiver. A picture on the TV screen is represented by a circle in FIG. 2A. The circle in FIG. 2A is enlarged in the zoom mode as shown in FIG. 2B. Increased amount of vertical and horizontal deflection is required in the zoom mode.
FIG. 3 shows the change in the vertical sawtooth wave associated with the zoom mode. The sawtooth wave in conventional mode is indicated by solid lines. And the sawtooth wave in zoom mode is indicated by dashed lines. The zoom mode is enabled by using the sawtooth wave indicated by the dashed lines. The amplitude of the sawtooth wave needs to be adjusted to the required enlargement. A limiter circuit clamps the amplitude of the sawtooth wave in FIG. 3 to provide the adjustment.
FIG. 4 shows a limiter circuit according to a conventional art. The sawtooth wave shown in FIG. 3 is generated in a signal source 1 in FIG. 4. The sawtooth wave is led to an output terminal 3 through a resistance 2. A transistor 4 and a reference voltage source 5 for upper amplitude limiting and a transistor 6 and a reference voltage source 7 for lower voltage amplitude limiting are connected to the output terminal 3.
The reference voltage source 5 is set at a certain voltage so that the emitter voltage of the transistor 4 becomes a level indicated by a chain line B in FIG. 3. The reference voltage source 7 is set at a certain voltage so that the emitter voltage of the transistor 6 becomes a level indicated by a chain line C in FIG. 3. Herewith the sawtooth wave indicated by the dashed lines in FIG. 3 varies between the level indicated by the chain line B and the level indicated by the chain line C.
When the sawtooth wave without limited amplitude is used, the vertical deflection angle of an electron beam is extended to cause scattering of the electron beam at the upper edge and the lower edge, resulting in abnormal brightening on the screen. Thus the amplitude of the sawtooth wave is limited to prevent the abnormal brightening on the screen.
Also, there is a problem that the display becomes distorted at the upper end and the lower end of the TV screen, due to the limiter circuit. This problem will be explained referring to FIG. 5. While horizontal lines in FIG. 5 should be displayed as equidistant parallel lines, the distances between the horizontal lines near the upper edge and the lower edge of the TV screen are narrowed. The cause is attributed to the clamping operation of the limiter circuit shown in FIG. 4.
FIG. 6 shows an enlargement of a portion A in FIG. 3. The amplitude of the sawtooth wave shown by dashed line in FIG. 6 is supposed to be limited at the level shown by chain line B. The sawtooth wave is supposed to keep a form of straight line indicated by the dashed line in FIG. 6 until its amplitude is limited. In reality, however, the amplitude of the sawtooth wave is limited taking a form of curve indicated by a solid curving line in FIG. 6. This is caused by conductance in rising operation of the transistor 4 and the transistor 6 in the limiter circuit shown in FIG. 4. Thus a limiter circuit which can clamp the amplitude of the sawtooth wave keeping its waveform straight as indicated by the dashed line in FIG. 6 is needed.
This invention is made considering the problems addressed above. A circuit according to this invention includes a first differential amplifier to an input terminal of which an input signal is applied, a first transistor to a base of which the output of the first differential amplifier is applied, a first feedback path connected between an emitter of the first transistor and the other input terminal of the first differential amplifier, a first current source connected to the emitter of the first transistor, a second differential amplifier to an input terminal of which a bias voltage is applied, a second transistor to a base of which the output of the second differential amplifier is applied, a second feedback path connected between an emitter of the second transistor and the other input terminal of the second differential amplifier, a second current source connected to the emitter of the second transistor and a resistance connected between the emitter of the first transistor and the emitter of the second transistor, and provides output signals from collectors of the first and the second transistors.
Or a circuit according to this invention includes a first differential amplifier to an input terminal of which an input signal is applied, a first transistor to a base of which the output of the first differential amplifier is applied, a first feedback path connected between an emitter of the first transistor and the other input terminal of the first differential amplifier, a first current source connected to the emitter of the first transistor, a second differential amplifier to an input terminal of which a bias voltage is applied, a second transistor to a base of which the output of the second differential amplifier is applied, a second feedback path connected between an emitter of the second transistor and the other input terminal of the second differential amplifier, a second current source connected to the emitter of the second transistor and a resistance connected between the emitter of the first transistor and the emitter of the second transistor, and provides an output signal from a collector of the first transistor or from a collector of the second transistor.
Furthermore, a circuit according to this invention includes a sawtooth wave generation circuit, a first differential amplifier to an input terminal of which an output signal of the sawtooth wave generation circuit is applied, a first transistor to a base of which the output of the first differential amplifier is applied, a first feedback path connected between an emitter of the first transistor and the other input terminal of the first differential amplifier, a first current path connected to the emitter of the first transistor and through which a constant current flows, a second differential amplifier to an input terminal of which a bias voltage is applied, a second transistor to a base of which the output of the second differential amplifier is applied, a second feedback path connected between an emitter of the second transistor and the other input terminal of the second differential amplifier, a second current path connected to the emitter of the second transistor and through which a constant current flows and a resistance connected between the emitter of the first transistor and the emitter of the second transistor, and provides output signals from an output signal from collectors of the first and the second transistors or from either of the collectors of the first and the second transistors.